1. Field of the Invention
The present invention relates to a process for hydroprocessing a catalytically cracked or thermally cracked naphtha stream. More particularly, the invention relates to a process for cracking an olefinic naphtha using a zeolite catalyst to form a cracked product having a diminished total olefin concentration, and then hydroprocessing at least a portion of the cracked product in a manner that reduces the sulfur concentration while substantially retaining the olefin content in order to maintain octane.
2. Background of the Invention
The need for low emissions, high octane fuels has created an increased demand for light olefins for use in alkylation, oligomerization, MTBE and ETBE synthesis processes. In addition, a low cost supply of C.sub.2 to C.sub.4 olefins, particularly propylene, continues to be in demand to serve as feedstock for polyolefin, particularly polypropylene production. In parallel with this need, increasingly stringent regulations require motor fuels having a diminished concentration of sulfur and, to a lesser extent, olefins boiling in the gasoline boiling range (C.sub.4 and above).
It is well known that conventional fluid catalytic cracking ("FCC") processes can be adapted to increase product C.sub.2 to C.sub.4 olefin concentration. Some of the adaptations include dual risers, combinations of cracking and metathesis, and the use of zeolite catalysts. Hydroprocessing cracked naphtha formed in such processes typically results in a product having a diminished concentration of olefinic species and non-hydrocarbyl species such as sulfur-containing species, and an augmented concentration of saturated species. Relatively severe hydroprocessing conditions are generally required to substantially remove sulfur-containing species, particularly in the presence of olefinic species having more than four carbon atoms, and such severe hydroprocessing conditions are known to result in a substantial octane reduction in the hydroprocessed product.
There remains a need, therefore, for new processes for forming C.sub.2 to C.sub.4 olefins together with naphtha having a diminished concentration of sulfur-containing species, while maintaining a sufficient amount of C.sub.4 and larger olefins in the naphtha, preferably C.sub.5 and C.sub.6 olefins, to provide a relatively high octane.